SCR catalyst ammonia surface coverage estimation and control

ABSTRACT

Described herein are various embodiments of an apparatus, a system, and a methods for reducing NO x  emissions using ammonia storage on an SCR catalyst. For example, according to one embodiment, an apparatus for controlling an SCR system of an internal combustion engine system includes an ammonia storage module and a reductant dosing module. The ammonia storage module determines an ammonia storage surface coverage on an SCR catalyst of the SCR system and an ammonia compensation value based on one of an excess ammonia flow rate entering the SCR catalyst and an excess NO x  flow rate entering the SCR catalyst. The reductant dosing module that generates a reductant dosing command based on the ammonia compensation value.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 12/767,664, filed on Apr. 26, 2010, the contents of which areincorporated herein by reference in its entirety.

FIELD

This disclosure relates to exhaust gas after-treatment systems forinternal combustion engines, and more particularly to an apparatus,system and method for estimating and controlling ammonia surfacecoverage on a selective catalytic reduction (SCR) catalyst.

BACKGROUND

Exhaust emissions regulations for internal combustion engines havebecome more stringent over recent years. The regulated emissions of NOand particulates from internal combustion engines are low enough that inmany cases the emissions levels cannot be met with improved combustiontechnologies. Therefore, the use of aftertreatment systems on engines toreduce harmful exhaust emissions is increasing. For reducing NO_(x)emissions, NO_(x) reduction systems, including selective catalyticreduction (SCR) systems, are utilized to convert NO_(x) (NO and NO₂ insome fraction) to N₂ and other compounds. SCR systems utilize areductant (e.g., diesel exhaust fluid (DEF), typically ammonia, and anSCR catalyst to convert the NO_(x). Currently available SCR systems canproduce high NO_(x) conversion rates allowing the combustiontechnologies to focus on power and efficiency. However, currentlyavailable SCR systems also suffer from several drawbacks.

Most SCR systems generate ammonia to reduce NO_(x) present in theexhaust gas generated by the internal combustion engine. When just theproper amount of ammonia is available at the SCR catalyst under theproper conditions, the ammonia is utilized to reduce NO_(x). Due to theundesirability of handling pure ammonia, many systems utilize analternate compound such as urea, that vaporizes and decomposes toammonia before entering the SCR catalyst. Many SCR systems that utilizeurea dosing to generate ammonia depend upon the real-time delivery ofurea to the SCR catalyst as engine NO_(x) emissions emerge. Urea dosershave relatively slow physical dynamics compared to engine transients,such as mass flow, temperature, and emissions. Therefore, urea doserdynamics can substantially affect an SCR controls system, particularlyduring transient operating conditions. For example, based on theoperating conditions, the urea dynamics may result in an excess ofammonia causing ammonia slip out of the SCR catalyst or a deficiency ofammonia causing excess amounts of NO_(x) entering the atmosphere.

Some currently available SCR systems account for the dynamics of theurea dosing and the generally fast transient nature of internalcombustion engines by utilizing the inherent ammonia storage capacity ofmany SCR catalyst formulations. Certain currently available systemsdetermine whether the SCR catalyst is at an ammonia storing (adsorption)or ammonia ejecting (desorption) temperature. When the SCR catalyst isstoring ammonia, the system injects urea until the catalyst is full.When the SCR catalyst is ejecting ammonia, the system halts injectionand allows stored ammonia to release and reduce NO_(x). Presentlyavailable systems tracking the SCR catalyst temperature in this mannersuffer from a few drawbacks. For example, the amount of ammonia storedon the SCR catalyst varies with temperature. However, presentlyavailable systems assume a storage amount below a specified temperature,and zero storage above the specified temperature. Therefore, thecontrols may toggle significantly around the specified temperature,significantly overestimate ammonia storage capacity just below thespecified temperature, and significantly underestimate ammonia storagecapacity just above the specified temperature.

SUMMARY

The subject matter of the present application has been developed inresponse to the present state of the art, and in particular, in responseto the problems and needs in the art that have not yet been fully solvedby currently available exhaust gas after-treatment systems. Accordingly,the subject matter of the present application has been developed toprovide apparatus, systems, and methods for reducing NO_(x) emissionsusing ammonia storage on an SCR catalyst that overcomes at least someshortcomings of the prior art exhaust gas after-treatment systems.

According to one embodiment, an apparatus for controlling an SCR systemof an internal combustion engine system includes an ammonia storagemodule and a reductant dosing module. The ammonia storage moduledetermines an ammonia storage surface coverage on an SCR catalyst of theSCR system and ammonia compensation value based on one of an excessammonia flow rate entering the SCR catalyst and an excess NO_(x) flowrate entering the SCR catalyst. The reductant dosing module thatgenerates a reductant dosing command based on the ammonia compensationvalue. In some implementations, the ammonia storage module utilizes afeedforward model exclusively for determining the current ammoniasurface coverage and the ammonia compensation value.

In certain implementations of the apparatus, the ammonia storage moduleincludes a mode determination module that determines the operating modeof the SCR system as one of an ammonia adsorption mode, an ammoniadesorption mode, and a neutral mode based on a pre-calibrated zeroammonia slip threshold. The mode determination module determines the SCRsystem is operating in the ammonia adsorption mode when anammonia/NO_(x) ratio of exhaust gas entering the SCR catalyst is greaterthan the pre-calibrated zero ammonia slip threshold, the ammoniadesorption mode when the ammonia/NO_(x) ratio is less than thepre-calibrated zero ammonia slip threshold, and the neutral mode whenthe ammonia/NO_(x) ratio is equal to the pre-calibrated zero ammoniaslip threshold.

The ammonia storage module can also include an excess ammonia flow ratemodule that determines the excess ammonia flow rate when the modedetermination module determines the SCR system is operating in theadsorption mode. Further, when the mode determination module determinesthe SCR system is operating in the adsorption mode, the ammoniacompensation value is based at least partially (e.g., indirectly) on theexcess ammonia flow rate. The ammonia storage module can further includean ammonia adsorption mode module that estimates an ammonia storagesurface coverage of the SCR catalyst based on the excess ammonia flowrate and a current temperature of the SCR catalyst. In certainimplementations, the ammonia adsorption mode module includes storedsystem dynamic adsorption time constant values relative to excessammonia flow rates and SCR catalyst temperatures. The ammonia adsorptionmode module can estimate a rate of change of the ammonia storage surfacecoverage by comparing the excess ammonia flow rate and a currenttemperature of the SCR catalyst to the stored system dynamic timeconstant values. The ammonia storage surface coverage is based on amathematical integration of the estimated rate of change of the ammoniastorage surface coverage. In certain implementations, the stored ammoniastorage surface coverage values for SCR catalyst temperatures above hightemperature threshold are approximately zero percent such that theestimated ammonia storage surface coverage is automatically resetfollowing high exhaust temperature events.

In certain implementations of the apparatus, the ammonia storage moduleincludes an excess NO_(x) flow rate module that determines the excessNO_(x) flow rate when the mode determination module determines the SCRsystem is operating in the desorption mode. Furthermore, when the modedetermination module determines the SCR system is operating in thedesorption mode, the ammonia compensation value is at least partiallybased on the excess NO_(x) flow rate. The ammonia storage module canfurther include an ammonia desorption mode module that estimates anammonia storage surface coverage of the SCR catalyst based on the excessNO_(x) flow rate and a current temperature of the SCR catalyst. In someimplementations, the ammonia desorption mode module includes storedsystem dynamic desorption time constant values relative to excess NO_(x)flow rates and SCR catalyst temperatures. The ammonia desorption modemodule estimates a rate of change of the ammonia storage surfacecoverage by comparing the excess NO_(x) flow rate and a currenttemperature of the SCR catalyst to the stored system dynamic timeconstant values. The ammonia storage surface coverage is based on amathematical integration of the estimated rate of change of the ammoniastorage surface coverage

According to some implementations of the apparatus, the ammonia storagemodule further includes an ammonia storage control module thatdetermines the ammonia compensation value based on the ammonia storagesurface coverage estimated by the ammonia adsorption or desorption modemodules 340, 345, and a predetermined ammonia storage surface coveragethreshold or target. The ammonia compensation value is negative if theestimated ammonia storage surface coverage is greater than thepredetermined ammonia storage surface coverage threshold. In contrast,the ammonia compensation value is positive if the estimated ammoniastorage surface coverage is less than the predetermined ammonia storagesurface coverage threshold.

In certain implementations, the ammonia compensation value is an ammoniacompensation flow rate. The ammonia compensation flow rate can be basedon a difference between the determine ammonia storage surface coverageand a predetermined ammonia storage surface coverage threshold.

According to another embodiment, an SCR system includes a reductantdosing system that doses reductant into an exhaust gas stream. The SCRsystem also includes an SCR catalyst that receives the exhaust gasstream and is positioned downstream of the reductant dosing system. TheSCR catalyst stores thereon ammonia present in the exhaust gas stream.The system also includes a controller that controls a dosing rate ofreductant dosed into the exhaust gas stream based on an excess ammoniaflow rate in the exhaust gas stream in an ammonia storage adsorptionmode and an excess NO_(x) flow rate in the exhaust gas stream in anammonia storage desorption mode.

In certain implementations of the system, the controller estimates anammonia storage surface coverage of the SCR catalyst based on the excessammonia flow rate in the ammonia storage adsorption mode and the excessNO_(x) flow rate in the ammonia storage desorption mode. The controlleralso increases the dosing rate if the estimated ammonia storage surfacecoverage is less than a predetermined desired ammonia storage surfacecoverage threshold and decreases the dosing rate if the estimatedammonia storage surface coverage is more than the predetermined desiredammonia storage surface coverage threshold. The desired ammonia storagesurface coverage threshold can be dependent on an age of the SCRcatalyst.

According to some implementations of the system, the excess ammonia flowrate is based on the difference between an ammonia flow rate in theexhaust gas stream and the product of a NO_(x) flow rate in the exhaustgas stream and a zero slip ammonia/NO_(x) ratio. In contrast, the excessNO_(x) flow rate is based on the difference between a NO_(x) flow ratein the exhaust gas stream and the product of an ammonia flow rate in theexhaust gas stream and the zero slip ammonia/NO_(x) ratio.

In another embodiment, a method for controlling an SCR system of aninternal combustion engine system includes determining a nominalreductant dosing rate for the SCR system and determining an operatingmode of the SCR system as one of an adsorption and desorption mode. Ifthe determined operating mode of the SCR system is the adsorption mode,the method includes estimating an ammonia storage surface coverage ofthe SCR system based on an excess ammonia flow rate in an exhaust gasstream flowing through the SCR system. If the determined operating modeof the SCR system is the desorption mode, the method includes estimatingthe ammonia storage surface coverage of the SCR system based on anexcess NO_(x) flow rate in the exhaust gas stream. Further, if theestimated ammonia storage surface coverage is greater than a desiredammonia storage surface coverage threshold, the method includes reducingthe determined nominal reductant dosing rate. Contrastingly, if theestimated ammonia storage surface coverage is less than a desiredammonia storage surface coverage threshold, then the method includesincreasing the determined nominal reductant dosing rate.

In certain implementations, the determined operating mode of the SCRsystem is the adsorption mode if an ammonia/NO_(x) ratio in the exhaustgas stream is greater than a pre-calibrated zero slip ammonia/NO_(x)ratio for a given SCR catalyst temperature, and the desorption mode ifthe ammonia/NO_(x) ratio in the exhaust gas stream is less than thepre-calibrated zero slip ammonia/NO_(x) ratio for the given SCR catalysttemperature. The method can also include automatically resetting theestimated ammonia storage surface area to zero to account for highexhaust temperature events by estimating the ammonia storage surfacecoverage of the SCR system.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the subject matter of the present disclosureshould be or are in any single embodiment. Rather, language referring tothe features and advantages is understood to mean that a specificfeature, advantage, or characteristic described in connection with anembodiment is included in at least one embodiment of the presentdisclosure. Thus, discussion of the features and advantages, and similarlanguage, throughout this specification may, but do not necessarily,refer to the same embodiment.

Furthermore, the described features, advantages, and characteristics ofthe subject matter of the present disclosure may be combined in anysuitable manner in one or more embodiments. One skilled in the relevantart will recognize that the subject matter may be practiced without oneor more of the specific features or advantages of a particularembodiment. In other instances, additional features and advantages maybe recognized in certain embodiments that may not be present in allembodiments. These features and advantages will become more fullyapparent from the following description and appended claims, or may belearned by the practice of the subject matter as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the subject matter may be more readilyunderstood, a more particular description of the subject matter brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict only typical embodiments of the subject matter and arenot therefore to be considered to be limiting of its scope, the subjectmatter will be described and explained with additional specificity anddetail through the use of the drawings, in which:

FIG. 1 is a schematic block diagram of an internal combustion enginehaving an exhaust gas after-treatment system according to onerepresentative embodiment;

FIG. 2 is a schematic block diagram of a controller of an exhaust gasafter-treatment system according to one representative embodiment;

FIG. 3 is a schematic block diagram of an ammonia storage module of anexhaust gas after-treatment system according to one representativeembodiment;

FIG. 4 is an ammonia storage mode determination table of a modedetermination module according to one representative embodiment;

FIG. 5 is an adsorption mode ammonia storage table of an ammoniaadsorption mode module according to one representative embodiment;

FIG. 6 is a desorption mode ammonia storage table of an ammoniadesorption mode module according to one representative embodiment;

FIG. 7 is a schematic block diagram of an ammonia storage control tableof an ammonia storage control module according to one representativeembodiment; and

FIG. 8 is a schematic flow chart diagram of a method for estimating andcontrolling ammonia surface coverage on an SCR catalyst according to onerepresentative embodiment.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Appearances of the phrases “in one embodiment,” “in an embodiment,” andsimilar language throughout this specification may, but do notnecessarily, all refer to the same embodiment. Similarly, the use of theterm “implementation” means an implementation having a particularfeature, structure, or characteristic described in connection with oneor more embodiments of the present disclosure, however, absent anexpress correlation to indicate otherwise, an implementation may beassociated with one or more embodiments.

Furthermore, the described features, structures, or characteristics ofthe subject matter described herein may be combined in any suitablemanner in one or more embodiments. In the following description,numerous specific details are provided, such as examples of controls,structures, devices, algorithms, programming, software modules, userselections, hardware modules, hardware circuits, hardware chips, etc.,to provide a thorough understanding of embodiments of the subjectmatter. One skilled in the relevant art will recognize, however, thatthe subject matter may be practiced without one or more of the specificdetails, or with other methods, components, materials, and so forth. Inother instances, well-known structures, materials, or operations are notshown or described in detail to avoid obscuring aspects of the disclosedsubject matter.

FIG. 1 depicts one embodiment of an internal combustion engine system100. The main components of the engine system 100 include an internalcombustion engine 110, an exhaust gas after-treatment system 120 coupledto the engine, and a controller 130 in electrically communicable withthe engine and exhaust gas after-treatment system. The internalcombustion engine 110 can be a compression ignited internal combustionengine, such as a diesel fueled engine, or a spark-ignited internalcombustion engine, such as a gasoline fueled engine operated lean.

Within the internal combustion engine 110, the air from the atmosphereis combined with fuel to power the engine. Combustion of the fuel andair produces exhaust gas. At least a portion of the exhaust gasgenerated by the internal combustion engine 110 is operatively vented tothe exhaust gas after-treatment system 120. In certain implementations,the engine system 100 includes an exhaust gas recirculation (EGR) line(not shown) configured to allow a portion of the exhaust gas generatedby the engine to recirculate back into the engine for altering thecombustion properties of the engine 110.

Generally, the exhaust gas after-treatment system 120 is configured toremove various chemical compound and particulate emissions present inthe exhaust gas received from the engine 110. The exhaust gasafter-treatment system 120 includes an oxidation catalyst 140, aparticulate matter (PM) filter 150, and an SCR system 160. In an exhaustflow direction, indicated by directional arrows between the exhaust gasafter-treatment system components, exhaust may flow from the engine 110,through the oxidation catalyst 140, through the particulate filter 150,through the SCR catalyst system 160, and from the SCR catalyst systeminto the atmosphere. In other words, in the illustrated embodiment, thePM filter 150 is positioned downstream of the oxidation catalyst 140,and the SCR catalyst system 160 is positioned downstream of the PMfilter 140. In other embodiments, the components of the exhaust gasafter-treatment system 120 can be positioned in any of variousarrangements, and the system can include other components, such as anAMOX catalyst (not shown), or fewer components, such as without a PMfilter. Generally, exhaust gas treated in the exhaust gasafter-treatment system 120 and released into the atmosphere consequentlycontains significantly fewer pollutants, such as diesel particulatematter, NO_(x), hydrocarbons, such as carbon monoxide and carbondioxide, than untreated exhaust gas.

The oxidation catalyst 140 can be any of various flow-through oxidationcatalysts, such as diesel oxidation catalysts (DOC), known in the art.Generally, the oxidation catalyst 140 is configured to oxidize at leastsome particulate matter, e.g., the soluble organic fraction of soot, inthe exhaust and reduce unburned hydrocarbons and CO in the exhaust toless environmentally harmful compounds. For example, in someimplementations, the oxidation catalyst 140 may sufficiently reduce thehydrocarbon and CO concentrations in the exhaust to meet the requisiteemissions standards.

The PM filter 150 can be any of various particulate filters known in theart configured to reduce particulate matter concentrations, e.g., sootand ash, in the exhaust gas to meet requisite emission standards. The PMfilter 150 can be electrically coupled to a controller, such as thecontroller 130, that controls various operational characteristics of thePM filter, such as, for example, the timing and duration of filterregeneration events.

The SCR system 160 includes a reductant delivery system 162 and an SCRcatalyst 164 downstream of the reductant delivery system. The reductantdelivery system 160 is operable to inject or dose a reductant into theexhaust gas prior to the gas entering the SCR catalyst 164 (e.g., at alocation between the PM filter 150 and the SCR catalyst 164). Althoughnot shown, in some embodiments, the reductant delivery system 162includes a reductant source, pump, and delivery mechanism (e.g., areductant injector or doser). The reductant source can be a container ortank capable of retaining a reductant, such as, for example, ammonia(NH₃), urea, diesel fuel, or diesel oil. The reductant source is inreductant supplying communication with the pump, which is configured topump reductant from the reductant source to the delivery mechanism. Thedelivery mechanism is selectively controllable to inject reductantdirectly into the exhaust gas stream prior to entering the SCR catalyst164.

The injected reductant (or broken-down byproducts of the reductant, suchas when urea is reduced to form ammonia) reacts with NO_(x) in thepresence of the SCR catalyst 164 to reduce NO_(x) in the exhaust gas toless harmful emissions, such as N₂ and H₂O. The SCR catalyst 164 can beany of various catalysts known in the art. For example, in someimplementations, the SCR catalyst 164 is a vanadium-based catalyst, andin other implementations, the SCR catalyst is a zeolite-based catalyst,such as a Cu-Zeolite or a Fe-Zeolite catalyst. In one representativeembodiment, the reductant is aqueous urea and the SCR catalyst 164 is azeolite-based catalyst.

The exhaust gas after-treatment system 120 includes various sensors,such as temperature sensors, mass flow sensors, NO_(x) sensors, and thelike that are disposed throughout the exhaust gas after-treatment systemin any of various arrangements. For example, in the illustratedembodiment, the exhaust gas after-treatment system 120 includes anexhaust temperature sensor 170 upstream of the SCR catalyst 164 (e.g.,between the PM filter 150 and SCR catalyst), an exhaust temperaturesensor 172 downstream of the SCR catalyst, an exhaust NO_(x)concentration sensor 174 upstream of the SCR catalyst (e.g., at anoutlet of the engine 110), and an exhaust mass flow sensor 176positioned upstream of the SCR catalyst (e.g., at the outlet of theengine 110). The various sensors may be in electrical communication withthe controller 130 to monitor operating conditions and control theengine system 100, including the engine 110 and exhaust gasafter-treatment system 100.

The controller 130 controls the operation of the engine system 100 andassociated sub-systems, such as the engine 110 and exhaust gasafter-treatment system 120. The controller 130 is depicted in FIG. 2 asa single physical unit, but can include two or more physically separatedunits or components in some embodiments if desired. Generally, thecontroller 130 receives multiple inputs, processes the inputs, andtransmits multiple outputs. The multiple inputs may include sensedmeasurements from the sensors and various user inputs. The inputs areprocessed by the controller 130 using various algorithms, stored data,and other inputs to update the stored data and/or generate outputvalues. The generated output values and/or commands are transmitted toother components of the controller and/or to one or more elements of theengine system 100 to control the system to achieve desired results, andmore specifically, reduce desired exhaust gas emissions.

The controller 130 includes various modules for controlling theoperation of the engine system 100. For example, referring to FIG. 2,and according to one embodiment, the controller 130 includes severalmodules for controlling operation of the SCR system 160. Generally, incertain embodiments, the controller 130 controls operation of the SCRsystem 160 to provide efficient and responsive reduction of NO_(x)during transient and steady state operating conditions, a reduction inthe amount of reductant dosed over a given duty cycle, and a reductionof ammonia slip from the tailpipe. The controller 130 includes a nominalreductant dosing module 200, an ammonia storage module 210, and areductant dosing module 220. Generally, the modules are independentlyand/or cooperatively operated to achieve optimal NO_(x) conversionefficiency and speed on the SCR catalyst 164 while minimizing ammoniaslip and reductant consumption. The controller 130 is communicable indata receiving and/or transmitting communication with severalsub-systems of the engine system 100, such as engine controls and SCRsystem controls.

The nominal reductant dosing module 200 is operable to determine anominal reductant dosing rate 230 corresponding with a reductant dosingrate for achieving a desired NO_(x) reduction efficiency without ammoniastorage considerations and without ammonia slip. The desired NO_(x)reduction efficiency is associated with an amount of NO_(x) that shouldbe reduced from the exhaust gas stream on the SCR catalyst 164 toachieve a predetermined exhaust gas emissions limit. In certainembodiments, the nominal reductant dosing module 200 determines thenominal reductant dosing rate 230 based on the temperature of the SCRcatalyst 164 and the space velocity of the exhaust gas entering the SCRcatalyst.

In some embodiments, the nominal reductant dosing module 200 uses any ofvarious methods and models for determining the nominal reductant dosingrate 230. For example, in one embodiment, the nominal reductant dosingmodule 200 first determines a NO_(x) reduction requirement associatedwith the desired NO_(x) reduction efficiency. The NO_(x) reductionrequirement is based on the amount of NO_(x) present in the exhaust gasand can be expressed as the fraction of the NO_(x) in the exhaust gasstream to be reduced. In certain implementations, the amount of NO_(x)present in the exhaust gas stream can be measured using a NO_(x) sensor,such as NO_(x) sensor 174. Alternatively, or additionally, in someimplementations, the amount of NO_(x) present in the exhaust gas streamcan be estimated based on the operating conditions of the engine andcomparing the conditions against a stored operating map containingpredetermined exhaust NO_(x) levels for various operating conditions.

Generally, the nominal reductant dosing module 200 calculates thereductant dosing rate necessary to achieve the determined NO_(x)reduction requirement and assign the calculated rate as the nominalreductant dosing rate 230. The nominal reductant dosing rate 230 can bedetermined using any of various conventional feedforward or feedbackmethodologies, such as described in U.S. patent application Ser. No.12/112,678, filed Apr. 30, 2008, which is incorporated herein byreference.

The ammonia storage module 210 of the controller 130 is operable todetermine an ammonia compensation value 240 representing an amount ofammonia added to or omitted from the nominal reductant dosing rate 230to achieve a desirable or desired ammonia storage surface coveragethreshold on the SCR catalyst 164. As described in more detail below,the illustrated embodiments utilize a feedforward model for determiningthe ammonia compensation value 240. For example, the controller 130 doesnot require the results from a NO_(x) sensor at the outlet of the SCR tocontrol the SCR system 160. As used herein, ammonia storage surfacecoverage is defined as the amount of surface area of the bed of the SCRcatalyst 164 covered by stored ammonia. In certain embodiments, theammonia storage surface coverage is expressed as a percentage of themaximum ammonia storage capacity of the SCR catalyst 164.

As discussed above, ammonia stored on the surface of the SCR catalystcan be utilized to compensate for time delays associated with thereductant dosing system 162 and the fast transient nature of the engine110. For example, if the operational conditions of the engine 110 changesuch that the amount of NO_(x) in the exhaust gas drops rapidlyresulting in excess ammonia in the exhaust gas, the excess exhaustammonia can be stored on the surface of the SCR catalyst 164 rather thanslip from the system. In contrast, if the operational conditions of theengine 110 change such that the amount of NO_(x) in the exhaust gasincreases rapidly resulting in excess NO_(x) in the exhaust gas, theammonia stored on the surface of the SCR catalyst can be used to reducethe excess NO_(x). Accordingly, the desired ammonia storage surfacecoverage threshold on the SCR catalyst 164 generally is more than 0% andless than 100% of the maximum ammonia storage capacity of the SCRcatalyst. In certain implementations, the desired ammonia storagesurface coverage threshold is between approximately 20% and 60% of themaximum ammonia storage capacity of the SCR catalyst 164. For certainapplications, target ammonia storage surface coverage thresholds withinthis range provide an optimal combination of maximum NO_(x) conversionefficiency and minimal NH₃ slip.

The desired ammonia storage surface coverage threshold is determinedbased on any of various factors, such as the temperature of the exhaustgas, age of the SCR catalyst, temperature of the SCR catalyst bed,exhaust mass flow rate, and type of engine (e.g., duty cycle of engine).In one exemplary implementation, the target ammonia surface coverage isprogrammed in the controller 130 as a function of SCR catalyst bedtemperature and exhaust mass flow rate.

Based on the nominal reductant dosing rate 230 and ammonia compensationvalue 240, the reductant dosing module 220 generates a reductant dosingcommand 250 representing the combined nominal reductant dosing rate andammonia compensation value. The reductant dosing command 250 iselectrically communicated to the reductant dosing system 162, whichinjects reductant into the exhaust gas stream at a rate correspondingwith the reductant dosing command. In certain conditions, the ammoniacompensation value 240 is a negative value such that the reductantdosing command 250 corresponds to a reductant dosing rate that is lessthan the nominal reductant dosing rate 230. In contrast, in otherconditions, the ammonia compensation value 240 is a positive value suchthat the reductant dosing command 250 corresponds to a reductant dosingrate that is more than the nominal reductant dosing rate 230.Alternatively, in some conditions, the ammonia compensation value 240 isapproximately zero such that the reductant dosing command 250corresponds to a reductant dosing rate that is the same as the nominalreductant dosing rate 230.

Referring to FIG. 3, the ammonia storage module 210 includes a modedetermination module 300 operable to determine whether the SCR system160 is operating in an ammonia storage adsorption mode, an ammoniastorage desorption mode, or a neutral mode. Generally, the modedetermination module 300 determines the SCR system 160 is operating inthe ammonia storage adsorption mode when the mode determination moduledetermines the ammonia storage surface coverage is increasing over time,the ammonia storage desorption mode when the mode determination moduledetermines the ammonia storage surface coverage is decreasing over time,and the neutral mode when the mode determination module determines theammonia storage surface coverage is not changing over time. The ammoniastorage module 210 receives as inputs an SCR catalyst temperature 305,an ammonia/NO_(x) ratio 310, and an exhaust mass flow rate 313. The SCRcatalyst temperature 305 is the measured or estimated temperature of thebed of the SCR catalyst 164. In one implementation, the SCR catalysttemperature 305 is measured using an SCR catalyst bed temperaturesensor. In another implementation, the SCR catalyst temperature 305 isestimated using any of various catalyst bed temperature estimationmethods known in the art, such as utilizing the exhaust temperaturedifference measured between temperature sensors 170 and 172.

The ammonia/NO_(x) ratio 310 represents the ammonia/NO_(x) ratio in theexhaust gas stream at the inlet of the SCR catalyst 164. Generally, theammonia/NO_(x) ratio 310 is determined based on a comparison between ameasured or estimated value of the amount of NO_(x) and the amount ofammonia present in the exhaust gas stream upstream of the SCR catalyst164. In certain implementations, the amount of NO_(x) in the exhaust gasstream is estimated using engine calibration tables. In otherimplementations, the amount of NO_(x) in the exhaust gas is measuredusing one or more NO_(x) sensors, such as NO_(x) sensor 174. In certainimplementations, the amount of ammonia in the exhaust gas stream isbased on the amount of ammonia injected into the exhaust gas stream bythe reductant dosing system 162 and/or the amount of ammonia measured inthe exhaust gas stream by one or more ammonia concentration sensors (notshown).

The SCR catalyst temperature 305 and ammonia/NO_(x) ratio 310 arecompared against a predetermined ammonia storage mode determinationtable 315 stored on the mode determination module 300 to determine whichof the adsorption and desorption modes, if any, under which the SCRsystem 160 is operating. Referring to FIG. 4, the ammonia storage modedetermination table 315 includes a pre-calibrated zero slip threshold400 representing the ammonia/NO_(x) ratio in the exhaust gas at which noammonia is being adsorbed (i.e., stored) or desorbed (i.e., depleted).In other words, the zero slip threshold 400 represents theammonia/NO_(x) ratio resulting in the NO_(x) reduction requirement beingachieved by just the ammonia in the exhaust gas stream without ammoniastorage and without ammonia slip. As defined herein, the SCR system 160is operating in a neutral mode when the ammonia/NO_(x) ratio is equal tothe zero slip threshold.

At a given SCR catalyst temperature, exhaust gas with an ammonia/NO_(x)ratio above (i.e., greater than) the zero slip threshold 400 indicatesthat excess ammonia is present in the exhaust gas, a part of which willbe stored on the SCR catalyst 164. Accordingly, for ammonia/NO_(x)ratios above the zero slip threshold 400, the SCR system 160 isoperating in an ammonia adsorption mode. In contrast, at a given SCRcatalyst temperature, exhaust gas with an ammonia/NO_(x) ratio below(i.e., less than) the zero slip threshold 400 indicates a shortage ofammonia is present in the exhaust gas, which results in stored ammoniabeing used for NO_(x) reduction. Therefore, for ammonia/NO_(x) ratiosbelow the zero slip threshold 400, the SCR system 160 is operating in anammonia desorption mode.

The ammonia storage module 210 also includes an excess ammonia flow ratemodule 320 and excess NO_(x) flow rate module 325. The excess ammoniaflow rate module 320 is operable to determine an excess ammonia flowrate 330 representing the flow rate of excess ammonia in the exhauststream at a location upstream of the SCR catalyst 164. Similarly, theexcess NO_(x) flow rate module 325 is operable to determine an excessNO_(x) flow rate 335 representing the flow rate of excess ammonia in theexhaust stream at a location upstream of the SCR catalyst 164.Generally, the excess ammonia flow rate module 320 determines the excessammonia flow rate 330 by multiplying the measured or estimated exhaustflow rate by the difference between the estimated or measured amount ofammonia in the exhaust stream and the product of the estimated ormeasured amount of NO_(x) in the exhaust stream and the zero slipammonia/NO_(x) ratio (i.e.,NH_(3(excess flow rate))=Exhaust_((flow rate))[NH₃−(NO_(x)*ANR_(zero slip))]).Likewise, the excess NO_(x) flow rate module 325 determines the excessNO_(x) flow rate 335 by multiplying the measured or estimated exhaustflow rate by the difference between the estimated or measured amount ofNO_(x) in the exhaust stream and the product of the estimated ormeasured amount of ammonia in the exhaust stream and the zero slipammonia/NOx ratio (i.e.,NO_(x(excess flow rate))=Exhaust_((flow rate))[NO_(x)−(NH₃*ANR_(zero slip))]).

The excess ammonia and NO_(x) flow rate modules 320, 325 and selectivelyoperable based on whether the SCR system 160 is operating in theadsorption or desorption modes, respectively, as determined by the modedetermination module 300. For example, if the mode determination module300 determines the SCR system 160 is operating in the ammonia adsorptionmode, then the excess ammonia flow rate module 320 is activated todetermine the excess ammonia flow rate 330 while the excess NO_(x) flowrate module 325 is inactive. Alternatively, if the mode determinationmodule 300 determines the SCR system 160 is operating in the ammoniadesorption mode, then the excess NO_(x) flow rate module 325 isactivated to determine the excess NO_(x) flow rate 335 while the excessammonia flow rate module 320 is inactive.

Referring again to FIG. 3, the ammonia storage module 210 includes anadsorption mode module 340 and a desorption mode module 345. Whenactivated, each ammonia adsorption and desorption mode module 340, 345is operable to determine an ammonia storage surface coverage 350 of theSCR catalyst 164. The ammonia adsorption mode module 340 is activatedwhen the SCR system 160 is operating in the adsorption mode as discussedabove and the ammonia desorption mode module 345 mode module 345 isactivated when the SCR system is operating in the desorption mode asdiscussed above. The ammonia adsorption and desorption mode modules 340,345 include respective predetermined adsorption and desorption modeammonia storage tables 355, 360 stored thereon.

Generally, the ammonia storage surface coverage 350 is determined bycomparing the excess ammonia and NO_(x) flow rates 330, 335 and currentSCR catalyst bed temperature against the predetermined adsorption anddesorption mode ammonia storage tables 355, 360, respectively, todetermine respective adsorption and desorption time constant valuesassociated with the ammonia storage surface condition of the SCRcatalyst. In certain implementations, each adsorption time constantvalue in the table 355 represents the time required for the SCR catalyst164 (starting from zero ammonia storage surface coverage) to reach amaximum ammonia storage surface coverage at the excess ammonia flow rateand SCR catalyst bed temperature associated with each adsorption timeconstant value. Similarly, each desorption time constant value in thetable 360 represents the time required for the SCR catalyst 164(starting from a maximum ammonia storage surface coverage) to reach zeroammonia storage surface coverage at the excess NO_(x) flow rate and SCRcatalyst bed temperature associated with each desorption time constantvalue.

In one embodiment shown in FIG. 5, the adsorption mode ammonia storagetable 355 includes predetermined ammonia storage surface data 400 (e.g.,time constants) plotted against excess ammonia flow rate values and SCRcatalyst temperature values. In one specific embodiment, as an exampleonly, the adsorption mode ammonia storage table 355 includes thefollowing pre-determined adsorption time constants for the associatedexcess ammonia flow rates (g/hr) and an SCR catalyst bed temperature of150° C.: 15,000 (0.5 g/hr), 9564.8 (5 g/hr), 2391.2 (20 g/hr), 797.1 (60g/hr), 191.3 (250 g/hr), and 47.8 (1,000 g/hr). As another example, thetable 355 can include the following pre-determined adsorption timeconstants for the associated excess ammonia flow rates (g/hr) and an SCRcatalyst bed temperature of 400° C.: 15,000 (0.5 g/hr), 29.3 (5 g/hr),7.3 (20 g/hr), 2.4 (60 g/hr), 0.6 (250 g/hr), and 0.1 (1,000 g/hr).

When activated, the ammonia adsorption mode module 340 receives theexcess ammonia flow rate 330 from the excess ammonia flow rate module320, along with the current temperature of the SCR catalyst 164. Usingthe table 355, the ammonia adsorption mode module 340 determines thepredetermined ammonia storage surface time constant corresponding withthe received excess ammonia flow rate 330 and current SCR catalyst bedtemperature.

Each time constant is a predetermined value that captures thefundamental response time of the SCR catalyst to a perturbation (e.g.,excess NO_(x) or excess NH₃) upstream of the SCR catalyst by treatingthe system as a first order dynamic system. Moreover, each time constantcorresponds directly with a predetermined rate of increase of theammonia storage surface coverage. Accordingly, the ammonia adsorptionmode module 340 determines the rate of increase of the ammonia storagesurface coverage that corresponds with the determined time constant. Thedetermined time constant is used to calculate an ammonia storage surfacecoverage on the SCR catalyst 164 according to the following formula:ASSC_(t)=ASSC_(t-1)+[ASSC_(ROC)*Δt], where ASSC_(t) is the ammoniastorage surface coverage at time t, ASSC_(t-1) is a previouslydetermined ammonia storage surface coverage at some time t−1 before timet, ASSC_(ROC) is the rate of change of the ammonia storage surface, andΔt is the difference between time t and time t−1. The ammonia storagesurface coverage value calculated using the above formula is then set bythe ammonia adsorption mode module 340 as the current ammonia storagesurface coverage 350.

As shown in FIG. 6, the desorption mode ammonia storage table 360 isused by the ammonia desorption mode module 345 in a manner similar tothe adsorption mode ammonia storage table 355 to determine the currentammonia storage surface coverage except instead of counting up, itcounts down the ammonia surface coverage on the SCR by comparing theexcess NO_(x) flow rate 335 against the data. More specifically, theammonia desorption mode module 345 uses the same formula used by theammonia desorption mode module 345 except the ASSC_(ROC) determined fromthe time constant taken from the table 360 is a rate of decrease, whilethe ASSC_(ROC) determined from the time constant taken from the table355 is a rate of increase. In one specific embodiment, as an exampleonly, the desorption mode ammonia storage table 360 includes thefollowing pre-determined desorption time constants for the associatedexcess NO_(x) flow rates (g/hr) and an SCR catalyst bed temperature of170° C.: 10,216 (25 g/hr), 3405.3 (75 g/hr), 1702.7 (150 g/hr), 851.3(300 g/hr), 425.7 (600 g/hr), and 212.8 (1,200 g/hr). As anotherexample, the table 360 can include the following pre-determineddesorption time constants for the associated excess NO_(x) flow rates(g/hr) and an SCR catalyst bed temperature of 400° C.: 316.9 (25 g/hr),105.6 (75 g/hr), 52.8 (150 g/hr), 26.4 (300 g/hr), 13.2 (600 g/hr), and6.6 (1,200 g/hr).

The configuration and operation of the ammonia adsorption and desorptionmode modules 340, 345 provide an accurate estimate of ammonia storagesurface coverage on the SCR catalyst 164 without requiring feedback fromsensors placed downstream of the SCR catalyst. Moreover, because dynamictime constants of the SCR system 160 used to determine ammonia storagesurface coverage are independent of downstream conditions, the presenceor absence of an AMOX catalyst downstream of the SCR catalyst 164 doesnot affect the accuracy and efficacy of the modules 340, 345.

SCR catalyst dynamics increase rapidly with temperature. Thepredetermined catalyst dynamics time constants account for this physicalrelationship by being significantly small values (e.g., on the order ofa second or less). Because of the relatively small values for thecatalyst dynamics time constants, the ammonia adsorption and desorptionmode modules 340, 345 automatically reset the estimated amount ofammonia stored on the SCR catalyst 164 to zero following high exhausttemperature events. Generally, high exhaust temperature events caused byoperation of the engine 110 or exhaust after-treatment system 120 (e.g.,PM filter regeneration) causes most, if not all, of the ammonia storedon the SCR catalyst 164 to be desorbed or removed from the SCR catalyst.For proper operation of the SCR system 160, the complete release ofammonia from the SCR catalyst 164 due to a high exhaust temperatureevent must be taken into account by effectively resetting the ammoniastorage surface coverage to zero or 0%.

In conventional SCR systems, the ammonia storage surface coverage of theSCR catalyst set independently of the ammonia storage surface coveragecalculation. In other words, for conventional SCR systems, regardless ofthe estimated amount of ammonia stored on the SCR catalyst, if a hightemperature event is detected, the amount of ammonia stored on the SCRcatalyst for purposes of controlling the system operation is set to zeroaccording to logic independent of the ammonia storage surface coverageestimation logic.

In contrast to conventional systems, the resetting of the ammoniastorage surface coverage due to a high temperature event is built intothe logic for estimating the ammonia storage surface coverage. Morespecifically, as discussed above, because of the unique method ofcomparing the excess ammonia and NO flow rates to SCR catalysttemperature to estimate the ammonia storage surface coverage, thepredetermined ammonia storage surface coverage data 400, 500 accountsfor extreme exhaust temperature events. In other words, the ammoniastorage surface coverage value at extreme high exhaust temperaturestaken from the tables 355, 360 is at or near zero. In this manner, anautomatic SCR catalyst reset function is incorporated into theestimation of the ammonia storage surface coverage for extremetemperature events.

The ammonia storage module 210 additionally includes an ammonia storagecontrol module 365. As shown, the ammonia storage module 210 determinesthe ammonia compensation value 240 based on the ammonia storage surfacecoverage 350 (and the SCR catalyst temperature used to determine theammonia storage surface coverage) received from a respective one of theammonia adsorption and desorption mode modules 340, 345. The ammoniacompensation value 240 is determined by comparing the current ammoniastorage surface coverage 350 and the operating time of the SCR catalyst164 against at least one of a plurality of predetermined ammonia storagecontrol tables 370 stored on the ammonia storage control module 365. Asrepresented in FIG. 7, each table 370 includes a pre-calibrated desiredammonia storage surface coverage threshold 700 at a specific anddifferent SCR catalyst temperature. In certain implementations, thedesired ammonia storage surface coverage threshold 700 is lower forhigher SCR catalyst temperatures, and higher for lower SCR catalysttemperatures. This allows the SCR system 160 to more effectively reduceNO_(x) emissions in low temperature duty cycles by virtue of storing ahigh amount of ammonia on the SCR catalyst.

The ammonia storage control module 365 selects an ammonia storagecontrol table 370 of the plurality of tables 370 corresponding with theSCR catalyst temperature used to determine the time constants from table355 and/or table 36, which are used to estimate the current ammoniastorage surface coverage 350. The ammonia storage surface coverage 350and elapsed operating time of the SCR catalyst 164 are compared againstthe target ammonia storage surface coverage threshold 700 of theselected table 370. In some instances, there may not be a table 370having the SCR catalyst temperature. For example, the SCR catalysttemperature may be more than an SCR catalyst temperature associated withone table 370, but less than the next closest SCR catalyst temperaturefor which a table is available. Accordingly, for SCR catalysttemperatures falling between two SCR catalyst temperatures for whichtables are available, the ammonia storage control module 365 may beconfigured to determine the ammonia storage surface coverage using thedata provided by the two tables and common interpolation techniques.

Ammonia storage surface coverage values above (i.e., greater than) thedesired ammonia storage surface coverage threshold 700 indicate that alarger than desired portion of the surface of the SCR catalyst iscovered by stored ammonia. Accordingly, if the ammonia storage surfacecoverage 350 is above the desired ammonia storage surface coveragethreshold 700, then the dosing rate of reductant should be reduced tofacilitate desorption of the excess ammonia stored on the SCR catalyst164 and a reduction of the ammonia storage surface coverage. Generally,the ammonia storage control module 365 reduces the reductant dosing rateuntil the ammonia storage surface coverage meets the desired ammoniastorage surface coverage threshold 700.

In contrast, ammonia storage surface coverage values below (i.e., lessthan) the desired ammonia storage surface coverage threshold 700indicate that a smaller than desired portion of the surface of the SCRcatalyst is covered by stored ammonia. Accordingly, if the ammoniastorage surface coverage 350 is below the desired ammonia storagesurface coverage threshold 700, then the dosing rate of reductant shouldbe increased to facilitate adsorption of excess ammonia in the exhaustgas on the SCR catalyst 164 and an increase of the ammonia storagesurface coverage. Generally, the ammonia storage control module 365increases the reductant dosing rate until the ammonia storage surfacecoverage meets the desired ammonia storage surface coverage threshold700.

The ammonia storage control module 365 generates the ammoniacompensation value 240 based on whether reductant dosing should beincreased or decreased. As discussed above, the ammonia compensationvalue 240 represents an amount of ammonia added to or subtracted fromthe nominal reductant dosing rate 230 to achieve the desired ammoniastorage surface coverage threshold 700. Accordingly, in certainimplementations, the ammonia compensation value 240 is expressed as apositive reductant dosing rate if ammonia should be added, a negativereductant dosing rate if ammonia should be subtracted, and a zero amountif ammonia does not need to be added or subtracted (e.g., if the ammoniastorage surface coverage 350 is equal to the desired ammonia storagesurface coverage threshold 700).

The rate (e.g., speed and efficiency) at which the ammonia storagesurface coverage approaches the desired ammonia storage surface coveragethreshold 700 can be regulated by the ammonia storage control module365. For example, in certain applications, it may be desirable to meetthe desired ammonia storage surface coverage threshold 700 as rapidly aspossible, such as in applications involving highly transient operatingconditions. Alternatively, in applications involving consistentsteady-state operating conditions, the desired ammonia storage surfacecoverage threshold 700 need not be met at quickly. Accordingly, theammonia storage control module 365 also is operable to determine anappropriate magnitude of the ammonia compensation value 240 forapproaching the desired ammonia storage surface coverage threshold 700at a desirable rate. In certain implementations, the magnitude of theammonia compensation value is based on at least one of exhaust flowrate, SCR catalyst temperature, predicted future operating conditions ofthe engine 110, and the time constants obtained from the adsorption anddesorption mode ammonia storage tables 355, 360 as discussed above.

Based on the determined magnitude of the ammonia compensation value, adosing rate (e.g., dosing rate compensation) is selected that preventsan excessive increase in ammonia, which can lead to ammonia slip or areadeposit formation, and prevents an excessive decrease, which can lead tolower-than-desired NO_(x) conversion efficiencies. Despite the abilityto regulate the rate at which the ammonia storage surface coverageapproaches the desired ammonia storage surface coverage threshold 700 insome embodiments, in other embodiments, the ammonia storage controlmodule 365 preferably selects the magnitude of the ammonia compensationvalue 240 to approach the desired ammonia storage surface coveragethreshold 700 as quickly as possible without overshooting the target.

In certain implementations, the dosing rate compensation, whichcorresponds with the ammonia compensation value 240, is directlyproportional to the difference between the current storage surfacecoverage and the desired ammonia storage surface coverage (e.g., thedesired ammonia storage surface coverage threshold 700). Therefore, thedosing rate compensation corresponding with the ammonia compensationvalue 240 is automatically reduced to zero when the desired ammoniastorage surface coverage threshold 700 has been reached. In oneimplementation, the ammonia compensation value 240 as a measure of flowrate is calculated accorded to the formula:ACFR=ΔASSC*ASSC_(MAX)*MWA/[τ*V], where ACFR is the ammonia compensationflow rate, ΔASSC is the difference between the current ammonia storagesurface coverage 350 and the desired ammonia storage surface coveragethreshold 700, ASSC_(MAX) is the maximum ammonia storage surfacecoverage of the SCR catalyst, MWA represents the molecular weightadjustments, τ is the time constant determines from the tables 355, 360,and V is the SCR catalyst volume. Once the desired ammonia storagesurface coverage threshold 700 is reached, the ammonia compensation flowrate is set to zero and reductant is dosed at the nominal reductantdosing rate 230. In this manner, the maximum NOx conversion efficiencyof the SCR system 160 is achieved based on the current ammonia storagesurface coverage 350 and the quantity of reductant injected into theexhaust gas is reduced, which results in a cost savings.

As discussed above, the reductant dosing module 220 of the controller130 receives the ammonia compensation value 240 from the ammonia storagecontrol module 365 and combines it with the nominal reductant dosingrate 230 to generate the reductant dosing command 250.

According to one embodiment shown in FIG. 8, a method 800 for estimatingand controlling ammonia surface coverage on an SCR catalyst includesdetermining a nominal reductant dosing rate for an SCR system at 805.The nominal reductant dosing rate can be determined by operation of thenominal reductant dosing module 200, and/or using any of various methodsknown in the art. The method 800 also includes determining the operatingmode of the SCR system as one of an adsorption, a desorption, and aneutral mode at 810. In certain implementations, the mode determinationmodule 300 of the ammonia storage module 210 determines the operatingmode of the SCR system.

If decided at 815 that the SCR system is not operating in the adsorptionmode, the method 800 proceeds to decide if the SCR system is operatingin the neutral mode at 820. If the decision at 820 is positive, then areductant dosing command intended for transmission to a reductant dosingsystem is set to the nominal reductant dosing rate at 825. However, ifthe decision at 820 is negative (i.e., the SCR system is operating inthe desorption mode), then the method determines an excess NO_(x) flowrate at 830. In certain implementations, the excess NO_(x) flow rate canbe determined by the excess NO_(x) flow rate module 325. The excessNO_(x) flow rate determined at 830 is used to determine an ammoniastorage surface coverage at 835. In certain implementations, the ammoniadesorption mode module 345 is operated to determine the ammonia storagesurface coverage at 835.

Referring back in the method 800, if the decision at 815 is positive, anexcess ammonia flow rate is determined at 840. In certainimplementations, the excess ammonia flow rate can be determined by theexcess ammonia flow rate module 320. The excess ammonia flow ratedetermined at 840 is used to determine an ammonia storage surfacecoverage at 845. In certain implementations, the ammonia adsorption modemodule 340 is operated to determine the ammonia storage surface coverageat 845. It is decided at 850 whether the ammonia storage surfacecoverage determined at 835, or alternatively at 845, is greater than adesired ammonia storage surface coverage threshold. If the decision at850 is answered positively, then the nominal reductant dosing rate isdecreased at 855 and the reductant dosing command is set to thedecreased nominal reductant dosing rate at 860. However, if the decisionat 850 is answered negatively, then the nominal reductant dosing rate isincreased at 865 and the reductant dosing command is set to theincreased nominal reductant dosing rate at 870.

Many of the functional units described in this specification have beenlabeled as modules, in order to more particularly emphasize theirimplementation independence. For example, a module may be implemented asa hardware circuit comprising custom VLSI circuits or gate arrays,off-the-shelf semiconductors such as logic chips, transistors, or otherdiscrete components. A module may also be implemented in programmablehardware devices such as field programmable gate arrays, programmablearray logic, programmable logic devices or the like.

Modules may also be implemented in software for execution by varioustypes of processors. An identified module of executable code may, forinstance, comprise one or more physical or logical blocks of computerinstructions, which may, for instance, be organized as an object,procedure, or function. Nevertheless, the executables of an identifiedmodule need not be physically located together, but may comprisedisparate instructions stored in different locations which, when joinedlogically together, comprise the module and achieve the stated purposefor the module.

Indeed, a module of executable code may be a single instruction, or manyinstructions, and may even be distributed over several different codesegments, among different programs, and across several memory devices.Similarly, operational data may be identified and illustrated hereinwithin modules, and may be embodied in any suitable form and organizedwithin any suitable type of data structure. The operational data may becollected as a single data set, or may be distributed over differentlocations including over different storage devices, and may exist, atleast partially, merely as electronic signals on a system or network.

The schematic flow chart diagrams and method schematic diagramsdescribed above are generally set forth as logical flow chart diagrams.As such, the depicted order and labeled steps are indicative ofrepresentative embodiments. Other steps and methods may be conceivedthat are equivalent in function, logic, or effect to one or more steps,or portions thereof, of the methods illustrated in the schematicdiagrams. Additionally, the format and symbols employed are provided toexplain the logical steps of the schematic diagrams and are understoodnot to limit the scope of the methods illustrated by the diagrams.Although various arrow types and line types may be employed in theschematic diagrams, they are understood not to limit the scope of thecorresponding methods. Indeed, some arrows or other connectors may beused to indicate only the logical flow of a method. For instance, anarrow may indicate a waiting or monitoring period of unspecifiedduration between enumerated steps of a depicted method. Additionally,the order in which a particular method occurs may or may not strictlyadhere to the order of the corresponding steps shown.

The present subject matter may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

The invention claimed is:
 1. A controller for controlling a selectivecatalytic reduction (SCR) system of an internal combustion engine systemcomprising: a reductant dosing module configured to generate a reductantcommand based, at least in part, on a determined ammonia compensationvalue and output the generated reductant command to a reductant deliverysystem, the reductant delivery system injecting reductant into anexhaust gas stream at a rate based on the generated reductant command;and an ammonia storage module comprising: a mode determination moduleconfigured to determine an operating mode of the SCR system as one of anadsorption mode, a desorption mode, or a neutral mode; an ammoniaadsorption mode module or an ammonia desorption mode module configuredto receive a first ammonia storage surface coverage value and calculatea second ammonia storage surface coverage value based, at least in part,on the determined operating mode of the SCR system; and an ammoniastorage control module configured to determine the ammonia compensationvalue based, at least in part, on the calculated second ammonia storagesurface coverage value and output the determined ammonia compensationvalue to the reductant dosing module.
 2. The controller of claim 1,wherein determining the mode determination module is configured todetermine the operating mode of the SCR system based, at least in part,on a SCR catalyst temperature and an ammonia-to-NO_(x) ratio upstream ofa SCR catalyst.
 3. The controller of claim 2, wherein determining themode determination module is configured to determine the operating modeof the SCR system as one of the adsorption mode, the desorption mode, orthe neutral mode based on comparing the ammonia-to-NOx ratio upstream ofthe SCR catalyst to a zero slip threshold of an ammonia storage tablefor the SCR catalyst temperature.
 4. The controller of claim 2 whereinthe ammonia storage module further comprises: an excess ammonia flowrate module configured to determine an excess ammonia flow rateresponsive to the mode determination module determining the operatingmode of the SCR system is the adsorption mode; wherein the ammoniaadsorption mode module calculates the second ammonia storage surfacecoverage value based, at least in part, on the determined excess ammoniaflow rate.
 5. The controller of claim 4, wherein the ammonia adsorptionmode module calculates the second ammonia storage surface coverage valuebased, at least in part, on a time constant from an adsorption ammoniastorage table for the determined excess ammonia flow rate and a SCRcatalyst temperature.
 6. The controller of claim 5, wherein the timeconstant corresponds to a predetermined rate of increase of ammoniastorage surface coverage of a SCR catalyst.
 7. The controller of claim 2wherein the ammonia storage module further comprises: an excess NO_(x)flow rate module configured to determine an excess NO_(x) flow rateresponsive to the mode determination module determining the operatingmode of the SCR system is the desorption mode; wherein the ammoniadesorption mode module calculates the second ammonia storage surfacecoverage value is based, at least in part, on the determined excessNO_(x) flow rate.
 8. The controller of claim 7, wherein the ammoniadesorption mode module calculates the second ammonia storage surfacecoverage value based, at least in part, on a time constant from adesorption ammonia storage table for the determined excess NO_(x) flowrate and a SCR catalyst temperature.
 9. The controller of claim 8,wherein the time constant corresponds to a predetermined rate ofdecrease of ammonia storage surface coverage of a SCR catalyst.
 10. Thecontroller of claim 2, wherein the ammonia storage module is configuredto set the first ammonia storage surface coverage value to zeroresponsive to a high temperature event.
 11. A system comprising: an SCRsystem including: a reductant delivery system, and an SCR catalyst, anda controller configured to: receive a first ammonia storage surfacecoverage value indicative of an amount of ammonia stored on a surface ofthe SCR catalyst, determine an operating mode of the SCR system as oneof an adsorption mode, a desorption mode, or a neutral mode, calculate asecond ammonia storage surface coverage value based, at least in part,on the determined operating mode of the SCR system, determine an ammoniacompensation value based, at least in part, on the calculated secondammonia storage surface coverage value, generate a reductant commandbased, at least in part, on the determined ammonia compensation value,and output the generated reductant command to the reductant deliverysystem.
 12. The system of claim 11, wherein determining the operatingmode of the SCR system is based, at least in part, on a temperature ofthe SCR catalyst and an ammonia-to-NOx ratio upstream of the SCRcatalyst.
 13. The system of claim 12, wherein the ammonia-to-NOx ratioupstream is based, at least in part, on an amount of NOx determined froman engine calibration table.
 14. The system of claim 13, wherein theammonia-to-NOx ratio upstream is based, at least in part, on an amountof NOx received from a NOx sensor upstream of the SCR catalyst.
 15. Thesystem of claim 13, wherein the ammonia-to-NOx ratio upstream is based,at least in part, on an amount of ammonia dosed by the reductantdelivery system.
 16. The system of claim 13, wherein the ammonia-to-NOxratio upstream is based, at least in part, on an amount of ammoniareceived from an ammonia sensor upstream of the SCR catalyst.
 17. Amethod comprising: setting a first ammonia storage surface coveragevalue to zero responsive to a high temperature event; determining anoperating mode of an SCR system; calculating a second ammonia storagesurface coverage value based, at least in part, on the determinedoperating mode of the SCR system; determining an ammonia compensationvalue based, at least in part, on the calculated second ammonia storagesurface coverage value; generating a reductant command based, at leastin part, on the determined ammonia compensation value; and outputtingthe generated reductant command to a reductant delivery system, thereductant delivery system injecting reductant into an exhaust gas streamat a rate based on the generated reductant command.
 18. The method ofclaim 17, wherein the determined operating mode of the SCR system is oneof an adsorption mode or a desorption mode.
 19. A non-transitorycomputer-readable memory device storing instructions that, when executedby one or more processors, cause the one or more processors to performoperations comprising: interpreting a first ammonia storage surfacecoverage value; determining an operating mode of an SCR system as one ofan adsorption mode, a desorption mode, or a neutral mode; calculating asecond ammonia storage surface coverage value based, at least in part,on the determined operating mode of the SCR system; determining anammonia compensation value based, at least in part, on the calculatedsecond ammonia storage surface coverage value; generating a reductantcommand based, at least in part, on the determined ammonia compensationvalue; and outputting the generated reductant command to a reductantdelivery system, the reductant delivery system injecting reductant intoan exhaust gas stream at a rate based on the generated reductantcommand.
 20. The non-transitory computer-readable memory device of claim19 storing instructions that cause the one or more processors to performoperations further comprising: determining an excess ammonia flow rateresponsive to determining the operating mode of the SCR system is theadsorption mode; wherein calculating the second ammonia storage surfacecoverage value is based, at least in part, on the determined excessammonia flow rate.
 21. The non-transitory computer-readable memorydevice of claim 20 storing instructions that cause the one or moreprocessors to perform operations further comprising: determining anexcess NO_(x) flow rate responsive to determining the operating mode ofthe SCR system is the desorption mode; wherein calculating the secondammonia storage surface coverage value is based, at least in part, onthe determined excess NO_(x) flow rate.
 22. The non-transitorycomputer-readable memory device of claim 20, wherein determining theammonia compensation value is based on a desired ammonia storage surfacecoverage threshold of an ammonia storage control table selected based onthe calculated second ammonia storage surface coverage value.